Yes it would make a difference. A noticeable I'd assume. But there is one little catch. Most apps aren't designed to make use of this 800mhz ram yet. So I'd just stick with the 667mhz stuff for now.

An app doesnt need to be "designed" to make use of it. The application doesn't control how fast the date is sent from A to B, it will be sent at 800MHz regardless. It's not like a dual core processor or such where an application needs to be designed to make sure of a specific new technology. Thats like saying "Well, this program isn't designed for a 3.0GHz processor instead of a 2.4GHz, so there wont be any speed boost in upgrading"